Project Abstract Endothelial cells have important biomedical applications ranging from enhancing the patency of engineered vascular grafts and stents to promoting neovascularization in ischemic tissues. But their limited availability hinders the success of endothelial-cell-related technologies. The advances in stem cell technology offer a unique opportunity to address this issue. In particular, endothelial cells have been derived from human pluripotent stem cells (hPSCs), which can proliferate extensively and virtually provide an unlimited cell source. The recent success in making induced PSCs (iPSCs) offers additional advantages in providing immunologically compatible autologous hPSCs and enabling personalized therapy in the future. The key to exploiting this opportunity to advance endothelial-cell-related technologies is our ability to guide endothelial differentiation. In currently used methods, hPSCs are differentiated into hemangioblasts, which have both hematopoietic and endothelial potentials, followed by differentiation of hemangioblasts into endothelial cells in the presence of VEGF and fibronectin(FN)-coated surfaces. VEGF and FN are both essential for efficient endothelial differentiation, and they exhibit a synergistic effect due to the unique structure of FN, which has a cell-adhesive site and a VEGF-binding site positioned in nanoscale proximity. However, naturally-derived FN has batch-to-batch variations. In addition, covalently immobilized FN has structural change that blocks the cell- adhesive ligand; physically adsorbed FN preserves the active cell-adhesive domain but does not allow precise control of surface ligand density. Therefore, cell microenvironments created with FN are not tightly controlled, hampering consistent production of endothelial cells from stem cells. This problem can be addressed by using well-controlled synthetic materials that recapitulate the essential molecular structure underlying the synergistic effect of VEGF and FN in regulating endothelial differentiation. The objective of this application is to develop synthetic materials having the essential structural characteristics underlying the synergistic effect of VEGF and FN and to use these materials to guide endothelial differentiation of human iPSC-derived hemangioblasts. Our central hypothesis is that a cell-adhesive peptide and a VEGF-mimetic peptide fused to a pair of heterodimerizing coiled-coils, respectively, can be brought into nanoscale proximity through coiled-coil self- assembly and the materials functionalized with the heterodimer, together with soluble factors, will create well- controlled cell microenvironments for efficient and reproducible endothelial differentiation of iPSC-derived hemangioblasts. The specific aims are: (1) design, synthesize, characterize, and immobilize the polypeptides that self-assemble to present a cell-adhesive peptide and a VEGF-mimetic peptide in nanoscale proximity; (2) examine endothelial differentiation of human iPSC-derived hemangioblasts on the polypeptide-functionalized substrates. Successful completion of this project will result in well-controlled, biomimetic cell microenvironments for efficient and robust endothelial differentiation of iPSC-derived hemangioblasts.